It comes from Jemima Stratford QC. She has given a judgment on whether GCHQ can pass information onto the US, which is later used to facilitate drone strikes. The key section of the Stratford judgment is as follows:

Individuals participating in war are entitled to kill one another; they can invoke the defence of "combatant immunity". Both domestic and international law recognise the status of some individuals as "lawful combatants" engaged in "international armed conflict". Killing an individual outside of that framework is murder. Assisting in the killing of an individual outside of that framework is assisting in the act of murder. In our view, the drone strikes carried out by the CIA in Yemen and Pakistan (amongst other places) are not carried out in the context of an ‘international armed conflict’. The US is not at war with Yemen or Pakistan. The individuals who are targeted are not, therefore, "combatants" and their killers are not entitled to "combatant immunity". The US government has sought to justify the attacks by reference to the doctrine of "anticipatory self-defence"

Stratford warns that this US doctrine of "anticipatory self-defence" is not widely recognised under international law.* She therefore concludes that targeted assassinations (that’s what drone strikes involve) are illegal under British law as well:

The United Kingdom government does not and could not lawfully carry out drone strikes outside Afghanistan, such as those carried out by the US government in Yemen and Pakistan. Accordingly, in our view, if GCHQ transferred data to the NSA in the knowledge that it would or might be used for targeting drone strikes, that transfer is probably unlawful. The transfer or would be an accessory to murder for the purposes of domestic law.

This advice is devastating. It means that workers at GCHQ are potentially open to the charge of being accessories to murder. It is important to stress that there is still much uncertainty, and of course we don’t actually know exactly what information GCHQ gives (or does not give) to the US.

However, Stratford is a credible, highly regarded QC. If her opinion – give to the all-party parliamentary group on drones, chaired by the Labour MP Tom Watson – committee is right, it appears that the British government may be asking GCHQ employees to carry out illegal acts, which in turn could have very serious consequences.

There has been silence about this from all major politicians, from all parties. Yet drones (unmanned aircraft or missiles) are rapidly becoming one of the dominant forms of modern warfare. Killing is carried out from behind a screen, often thousands of miles away from the combat zone. We urgently need to work out our attitude to drones, what rules should apply, and to be clear about the legal situation surrounding them. Ministers can no longer avoid these grave issues of life and death.

* Stratfordwrites: "The doctrine of anticipatory self-defence, as argued by the US government, has not been widely accepted within international law. The doctrine of anticipatory self-defence provides that, where the target presents an ‘imminent’ or ‘immediate’ threat, a state may strike first in self-defence. In effect, the attacking party must strike or be struck. The US government relies on a broader formulation of that principle. They cannot know, or demonstrate, that the targets of any particular drone strike present an imminent threat to US interests. In effect, they rely on intelligence and other information to argue that the targets might present an imminent threat. That broader formulation of the doctrine has not yet become a part of the consensus of international law. Indeed, to the contrary, it was the rationale advanced by Israel in order to justify a pre-emptive bombing strike on an Iraqi nuclear reactor over 30 years ago. That justification was rejected by the Security Council.

The UK government has also rejected this formulation of the doctrine of anticipatory self-defence much more recently. In his written report to Prime Minister Tony Blair, when evaluating the lawfulness of the invasion of Iraq the Attorney General wrote: “…I am aware that the USA has been arguing for recognition of a broad doctrine of a right to use force to pre-empt danger in the future. If this means more than a right to respond proportionately to an imminent attack (and I understand that the doctrine is intended to carry that connotation) this is not a doctrine which, in my opinion, exists or is recognised in international law.”